Conventional hot dip galvanization consisting of dipping iron or steel articles in a molten zinc bath requires careful surface preparation, in order to assure adherence, continuity and uniformity of the zinc coating. A conventional method for preparing the surface of an iron or steel article to be galvanized is dry fluxing, wherein a film of flux is deposited on the surface of the article before dipping it in the zincbath. Accordingly, the article generally undergoes a degreasing followed by rinsing, an acid cleaning also followed by rinsing, and a final dry fluxing, i.e. the article is dipped in a fluxing bath and subsequently dried. The basic products employed in conventional fluxing are generally zinc and ammonium chlorides.
Several important problems are currently encountered in the batch hot dip galvanizing or general galvanizing industry:
Problem No 1:
It has been proved that adding 250 to 500 ppm Aluminum to a classic zinc bath has a benefic influence on several factors: thinner zinc layer on Si-rich steel (Si>0.28%), as well as better drainability of the molten zinc alloy.
However, it is also well known that galvanizers that have tried to galvanize material with conventional flux in zinc bath containing 200 to 500 ppm Al have been confronted with a problem.
In particular, some areas of the surface may not be covered, or not be covered in a sufficient manner, or the coating may show black spots or even craters, which give the article unacceptable finish and/or corrosion resistance. Thus, research has been carried out to develop a pre-treatment process and/or fluxes and/or additives in the molten zinc that are more adapted to galvanize with zinc alloy containing Al 200-500 ppm. Despite these efforts, when it comes to the galvanizing of iron or steel articles in zinc-aluminum baths in batch operation, i.e. the galvanizing of individual articles, the known fluxes are still not satisfactory.
Problem No 2:
In order to galvanize steel parts in a correct and safe way, different types of holes are necessary in the steel constructions or articles;                a. holes in order to let the molten zinc access to all the zones of the construction/article        b. holes necessary in order to allow air, gases due to the melting of the flux (NH4Cl, AlCl3, water) to escape. A lot of documents exist that explain the best procedures to place the holes and to size them.        
However in the daily production, it is unfortunately frequent that in some articles the holes are too small and/or badly positioned (see FIG. 1). In such conditions, an important quantity of liquid (fluxing bath) is trapped in the construction and once it comes in contact with the molten zinc bath, large amounts of gas are produced leading to an explosion with the projection of up to several kilograms of molten zinc in the air above the zinc bath's surface. The molten zinc that has been projected reaches parts of the article that have not yet been dipped in the molten zinc and will stick to them. Depending on the thickness of the article, the importance of the zinc splashes (how much g Zinc/m2) and the composition of the zinc bath, the flux layer can be destroyed leading to poor wetting of the molten zinc and resulting in ungalvanized zones! When the zinc bath contains from about 200 to about 500 ppm aluminum, this phenomenon is clearly worse than with lower aluminum contents. The presence of aluminum catalyses the quick burning of the flux layer and because these explosions cannot be completely avoided, it is a major problem of galvanizing with 200-500 ppm Al.
Problem No 3:
A good drying of the flux layer is necessary in order                to avoid explosions,        to allow a as high as possible dipping speed. A high dipping speed diminishes the risk of Liquid Metal Embrittlement (also called Liquid-Metal-Assisted-Cracking)        to minimize the production of ashes and to minimize the zinc use (kg zinc/ton material)        
The best case would be to bring the material to be galvanized at 100° C. as quickly as possible in order to make sure that all water has been evaporated and that the flux is not yet burned (damaged). In the daily practice of BHDG (Batch Hot Dip Galvanizing also called General Galvanizing) one is confronted with three factors:                a. The galvanizing of constructions made out of steel parts of different thickness. For example, a water tank for a farmer is made out steel plates and profiles of 5, 8 and 12 mm. After drying, the parts have different temperatures depending on their thickness: thinner parts are hotter and thicker parts are colder.        b. The number of positions in the dryer are limited usually to two positions thus in order to follow the production rhythm, higher air temperature and higher turbulence are required to achieve drying in a sufficiently short time,        c. Sometimes the production has to be stopped for 30 minutes (for example during lunch breaks), some dips can take 40 minutes to be galvanized and therefore some material already in the dryer may have to stay there for 3 hours in the longer case and in the shorter case for only 10 minutes!        
The consequences of these factors is that some parts (thin parts) may sometimes reach the air temperature used for the drying and begin to corrode heavier in the dryer and thicker parts can sometimes be too cold and be still wet and this can induce explosions as mentioned above when entering the molten zinc bath.
Problem No 4:
Some articles may only be dipped very slowly into the molten zinc because these articles are hollow and the size of the openings is limited as is the case for example with kettles for compressed air and with kettles for water under pressure. Because of the pressure requirements of such articles, smaller opening sizes are necessary and it takes sometimes up to 30 minutes to dip the kettle completely into the molten zinc. During this period, the molten zinc heats up the steel and this leads to the burning (melting and disappearing) of the flux layer before it comes in contact with the molten zinc.